In an electric brake system of a brake-by-wire type, a target brake pressure is calculated by using a driver input of, for example, a pedal travel, and various types of actuators generate a hydraulic pressure so as to obtain a target hydraulic pressure.
Such an electric brake system uses an electric motor as an actuator for generating a brake hydraulic pressure. The electric motor may use an interior permanent magnet (IPM) motor in which a permanent magnet is placed in a rotor and a coil is driven by an inverter (see, for example, Japanese Unexamined Patent Application Publication No. 2011-143850). The IPM motor can obtain a reluctance torque caused by a change in magnetic reluctance in a gap between a stator and a rotor in addition to a magnetic torque caused by a suction/repulsive force between the coil and the permanent magnet. Thus, the IPM motor is preferably used for applications requiring high-power performance, e.g., drive motors of electric vehicles such as hybrid vehicles. The IPM motor can also be used for motors for driving electric hydraulic pressure generators that generate hydraulic pressures in accordance with brake operations of drivers.
The torque of an IPM motor is a combined torque which is a combination of a magnetic torque and a reluctance torque, which are functions of advance values. The IPM motor needs to set a current phase such that the combined torque of the magnetic torque and the reluctance torque is at maximum.
To generate the above-described reluctance torque, an advance control that produces a mechanical offset (an advance amount) of a sensor attachment position is employed. This advance control is performed by mechanically offsetting an attachment position of, for example, a Hall sensor for specifying the position of the IPM motor. The offset that causes an appropriate advance angle is sufficiently considered. Specifically, an angle sensor is attached with an offset corresponding to an angle at which a reluctance torque is at maximum, thereby enabling generation of a maximum torque of the IPM motor.